Tension transients were recorded from fibres isolated from the tibialis anterior muscle of the frog during the rise of tetanic tension at 0.8-2.5 degrees C. The length of a central segment of the fibre was controlled by feed-back from a spot-follower device. Length steps complete within 0.2 ms were applied at one end of the fibre, and tension changes were recorded at the other end with a transducer having a natural frequency of 10.8 kHz. The tension transients measured during the rise of force showed the four phases characteristic of transients recorded during the plateau of a tetanus and during shortening. The extreme tension change reached during a length change was smaller for a given size of step during the rise of tension than at the plateau, but by less than in proportion to the developed force, suggesting that stiffness increases earlier than tension. Stiffness changes were further assessed by matching the tension records from one fibre with the responses of an analogue circuit (delay line) representing the mechanical properties of the fibre and force transducer. Stiffness derived from these comparisons varied in approximately the same proportion as stiffness assessed from the extreme tension change. During the rise of tension, there was a roughly constant lag of tension behind stiffness, ranging from 11 to 16 ms in different fibres. Steps applied during the latent period showed a lag of about 10 ms from the first increase of stiffness to the first appearance of tension. The partial recovery of tension immediately following the step, phase 2, was faster at the low tension levels early in the tetanus. The intermediate level, T2, to which tension recovers during phase 2 scaled in approximate proportion to the tension level immediately preceding the step. This result is unlike the relative decrease in T2 levels we have recently described for steps applied during steady shortening, and suggests that the increased stiffness-tension ratio seen during the rise of tetanic force is not due to shortening within the sarcomeres. The results can be explained if the attachment of cross-bridges in the rising phase takes place in two steps, the initial state of attachment resulting in the production of little or no tension. Several such schemes are considered.